Textile materials can be made into a variety of different physical forms, including fibers per se, a fiber assembly such as a fiber tow, mat, web, or the like. Textile materials are further exemplified in a variety of fibrous textile structures including traditional textile fibers and fabrics and microfibrous mats, webs, or the like.
The Textile Institute defines a fabric as "a manufactured assembly of fibers and/or yarns, which has substantial area in relation to its thickness and sufficient mechanical strength to give the assembly inherent cohesion". Traditional textile fabrics are most commonly woven or knitted, but the term includes assemblies produced by lace-making, tufting, felting, net-making, and so called nonwoven processes. The distinctive characteristics of a fabric arises from the manner in which the fibers are arranged in a sheet-like or planar structure. Woven and knitted fabrics are made by interlacing and interlooping of assemblies of filaments or fibers (monofilament, or multifilaments with staple yarns). On the other hand, traditional nonwoven fabrics are made by blending staple fibers with a polymeric binder material followed by bonding to form a web-like array of fibers or filaments or, alternatively, mechanically entangling the fibers such as by hydroentanglement or lock stitching (needle punching) to form a mechanically entangled web-like array. The web-like array can be made from fibers of discreet lengths (ranging from a few millimeters to a few meters) by a carding or wet laying process.
An alternative to traditional nonwoven processes for the production of microfibrous materials consists of laying or blowing filaments as they are melt extruded. The microfibrous material made by these latter processes are commonly known as spunbonded and meltblown nonwoven fabrics.
The term "fibrous material" as used herein refers to a multiplicity of randomly entangled self bonded polymeric precursor fibers or microfibers in the form or shape of a nonwoven generally planar panel, sheet, mat, web, batting, or the like. It will be apparent to persons ckilled in the art that the final shape of the randomly entangled self bonded polymeric precursor fibers or microfibers can be made to any desired specification, be it pillow shaped, rod like or the like. The fibrous material is produced using either the meltblown or spunbond process or some modification or combination thereof.
The exact final structural form of the microfibrous material can also be tailored by controlling the process depending upon the desired loft and density of the material required, i.e. a relatively thick, low density mat or a relatively thin, higher density mat. The fibrous material can also be in the form of a single ply mat or web, or a multiplicity of superimposed or stacked plies in the form of a high loft mat or batt-like structure. In other words, a spunbonded fabric can be defined generically as a continuous interconnected polymeric microfibrous fabric or material. A melt blown fabric is defined by a process in which extremely fine or "super fine" microdenier fibers of typically less than 10 microns in diameter are extruded under the influence of a dynamic flow of air and are then collected in the form of a microfibrous material on a screen or belt. As a result of the dynamic air flow, the fibers are drawn while they are still hot and tacky, so that there is obtained a difference in birefringence, crystallinity and molecular orientation as compared to conventionally spun fibers. The microdenier fibers produced in a meltblown process are much finer than those that can be produced by the traditional textile techniques of melt spinning or other traditional methods of spinning textile fibers. The meltblown microdenier fibers are bonded at the fiber to fiber intersections and contact points while the fibers are still tacky. Typical melt blown fibrous materials are usually thin, but it is possible by placing multiple extrusion heads in a close series relationship to build up an interconnected (bonded) mat or batt, for example, having a thickness of from 0.1 to 6 inches (2.5 to 150 mm), which is similar in thickness to the thick batts produced from textile fibers using a traditional air-laid process. The birefrigence of meltblown and spunbonded fibers is relatively low. Meltblown fibers exhibit a lower level of birefringence than the spunbonded fibers which in turn exhibit a lower level of birefringence than traditional spun and drawn textile fibers.
The extremely fine diameter of meltblown microfibers yields a microfibrous material with a surface area of from 4 to 20 times the surface area of similar weight fibrous material containing traditional textile fibers. This phenomenon greatly increases the thermal insulation property of a meltblown polymeric microfibrous material, but, at the same time, also greatly increases the flammability of the material. For this reason, a meltblown microfibrous material is usually covered or coated on the fiber surfaces by another ignition resistant material to reduce or prevent ignition.
Processes for preparing microfibrous materials from thermoplastic materials using a meltblown process have been described in publications such as Industrial and Engineering Chemistry, Vol. 48, No. 8 (1956), pages 1342-1346. Meltblown processes are also described in U.S. Pat. Nos. 2,374,540 and 3,532,800. Methods for preparing spunbonded articles are described in U.S. Pat. Nos. 3,379,811 and 3,502,763.
U.S. Pat. No. 4,118,531, which is incorporated herein by references, discloses meltblown webs that comprise a mixture of meltblown microfibers and crimped bulking fibers wherein the mixed fibers are used for thermal insulation. These webs are sold under the tradename Thinsulate.TM. by Minnesota Mining and Manufacturing Corporation and are generally used as insulation for clothing articles. These webs are not irradiated nor are they heat treated to render them carbonaceous and are therefore highly flammable.
As is well known, meltblown materials have found utility in a broad range of applications. For example, it is known to use polymeric meltblown filaments in the preparation of battery separators, cable wrap, capacitor insulation paper, as wrapping materials, clothing liners, diaper liners, in the manufacture of bandages and sanitary napkins, and the like.
U.S. Pat. Nos. 4,837,076, 4,879,168 by McCullough et al, which are herein incorporated by reference, disclose crimped, irreversibly heat set, carbonaceous fibers which are derived from oxidatively stabilized polyacrylonitrile fibers. These patents disclose heat treatment conditions suitable to permanently heat set the fibers in an inert atmosphere to make the fibers carbonaceous. These heat treatment conditions can be used in the process of the present invention. These fibers, however, are traditionally spun textile fibers made from acrylic (PAN) which have been oxidatively stabilized and which contain from about 5 to 20 percent by weight oxygen.
Exposing polymeric materials to ionizing radiation to alter their properties is known. Methods of radiation include X-rays, gamma-rays and electron beam (or E-Beam) radiation. These kinds of radiation are all essentially equivalent. Under exposure to radiation, free radicals or other reactive species are generated in the polymeric material. Ionizing radiation, e.g. from an electron beam generator is known to create many complex and sometimes competing reactions. For example, electron beam radiation is known to induce crosslinking of acrylonitrile.
The electron beam radiation treatment is most easily carried out at ambient temperature. There is no obstacle, however, to the radiation treatment of the fibrous material at an elevated temperature, provided that the temperature is maintained below the temperature at which the polymeric material degrades or deteriorates.
The term "carbonaceous material" used herein is understood to mean that the carbon content of the material is greater than about 65% and less than 92%, the oxygen content is greater than 0 and less than 2%, preferably less than 1%, the nitrogen content is from about 5 to 30%, and the specific resistivity of the carbonaceous material is greater than 10.sup.-1 ohm-cm., more specifically from about 10.sup.1 up to about 10.sup.-10 ohm-cm,
The term "carbonization" used herein is understood to mean that the carbon content of the irradiated acrylic polymeric precursor material has been increased as a result of an irreversible chemical reaction generally caused by heat treating the material in a nonoxidizing atmosphere to permanently heat set the material.
All percentages given herein are in percent by weight unless otherwise specified.